1. Field of the Invention
The invention relates to the field of cordless telephones.
2. Prior Art
Cordless telephone systems are well-known in the prior art. They typically include a base unit which connects to a standard alternating current (AC) power outlet and a telephone line. This base unit communicates through radio transmissions with a remote unit. The remote unit is generally a hand-held member through which calls are received and placed via the base unit. Within the confines of allowable transmission power, ranges of 500 feet or so are achieved. Transmissions from the remote unit to the base unit are made at approximately 49 mHz. This Federal Communications Commission (FCC) certified link includes five frequency modulated (FM) channels each spaced-apart by 30 kHz. The base unit transmits at approximately 1.7 mHz to the remote unit. This lower frequency signal is generally coupled into the AC wiring of the house or other building, using the wiring as an antenna.
With cordless telephones, the base unit is generally open to place calls for any remote unit tuned to its same frequency. Unfortunately, this arrangement allows the placement of calls by unauthorized remote units. In some cases, a special tone has been placed on the signal transmitted by the remote unit to the base unit to prevent unauthorized calls. If the base unit detects this tone, then it is assumed that an authorized call is being placed. These tones are in the frequency range of approximately 1 kc. Expensive filtering circuitry is required in the base unit to remove this tone frequency and prevent its transmission into the telephone line.
As will be seen, the present invention provides a plurality of special tones used to prevent unauthorized calls. These tones are at, or near, the lower limit of the audio range and can be easily removed from the audio signal without the expensive filters required by the prior art units.
Prior art remote units use two oscillators, one for providing the transmission frequency and the other for the local oscillator signal which is mixed with the received signal to provide the intermediate frequency (IF). The present invention employs a single crystal controlled oscillator which is used to provide both the transmitted frequency and as a local oscillator for the mixer. The single oscillator not only reduces the cost of the system but improves performance.
Several other improvements over prior art systems are disclosed. For example, during standby modes, the remote unit reduces its standby power requirements. This increases the useful life of the batteries. The physical layout of the remote unit provides improved isolation between the received and transmitted signals.